Artificial intervertebral disc - Patent Review 6315797

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BACKGROUND

1. Field of the Disclosure

The present disclosure generally relates to apparatus and techniques for treatment of spinal disorders, and, in particular, relates to an artificial intervertebral prosthesis which restores both the height and shape of the intervertebral disc space following the removal of a damaged or diseased intervertebral disc while maintaining the natural biomechanics of the spinal motion segment.

2. Description of the Prior Art

The objective in intervertebral disc replacement is to provide a prosthetic disc that combines both stability to support the high loads of the patient's vertebrae and flexibility to provide the patient with sufficient mobility. In attempting to strike this balance, generally, four basic types of artificial intervertebral discs for replacing a part or all of a removed disc have been developed, namely, elastomer discs, ball and socket discs, mechanical spring discs and hybrid discs. Elastomer discs typically include an elastomer cushion which is sandwiched, between lower and upper rigid endplates. The elastomer discs are advantageous in that the elastomer cushion functions similar in mechanical behavior to the removed intervertebral disc tissue. However, a disadvantage of this disc type is that the elastomer cushion experiences long term in-vivo problems stemming from microcracking, which detracts from its usefulness as a replacement option Furthermore, attachment of the elastomer cushion to the endplates presents additional difficulties. Examples of elastomer discs are disclosed in U.S. Pat. Nos. 5,702,450 to Bisserie; 5,676,792 to Ratron; 5,035,716 to Downey; 4,874,389 to Downey; and 4,863,477 to Monson. Ball and socket discs typically incorporate two plate members having cooperating inner ball and socket portions which permit articulating motion of the member! during movement of the spine. The ball and socket arrangement is adept in restoring "motion" of the spine, but, is poor in replicating the natural stiffness of the intervertebral disc. This low stiffness places detrimentally high loads on supporting ligaments and muscles, particularly, in movement involving torsional rotation of the spine. Dislocation and wear are other concerns with this disc type. Examples of ball and socket discs are disclosed in U.S. Pat. No. Nos.: 5,507,816 to Bullivant and 5,258,031 to Salib et al.

Mechanical spring discs usually incorporate one or more coiled springs disposed between metal endplates. The coiled springs preferably define a cumulative spring constant sufficient to maintain the spaced arrangement of the adjacent vertebrae and to allow normal movement of the vertebrae during flexion and extension of the spring in any direction. Disadvantages of the mechanical spring disc types involve attachment of the coiled springs to the metal end plates and associated wear at the attachment points. Furthermore, fibrous Issue growth or encroachment into the coiled springs presents additional difficulties. Examples of mechanical spring discs are disclosed in U.S. Pat. Nos.: 5,458,642 to Beer et al. and 4,309,777 to Patil.

The fourth type of artificial intervertebral disc, namely, the hybrid type incorporates two or more principles of any of the aforedescribed disc types. For example, one known hybrid disc arrangement includes a ball and socket set surrounded by an elastomer ring. This hybrid disc provides several advantages with respect to load carrying ability, but, is generally complex requiring a number of individual components. Furthermore, long term in vivo difficulties with the elastomer cushion remain a concern as well as wear of the ball and socket arrangement.

Another type of intervertebral disc prosthesis is disclosed in U.S. Pat. No. 5,320,644 to Baumgartner. With reference to FIGS. 1-3, the Baumgartner '644 device is a unitary intervertebral disc member 1 made from a strong, elastically deformable material. The disc member 1 has parallel slits 5 each arranged at a right angle to the axis of the disc member. The parallel slits 5 partially overlap one another to define overlapping regions 6 between adjacent slits. The overlapping regions 6 create a leaf spring effect for the transmission of forces from one vertebral attachment surface to the other. In regions of adjacent slits 5 where they do not overlap the spring action on the leaf springs 7 is interrupted by fixation zones 9 of solid prosthesis material. The forces acting on the intervertebral disc are transmitted from one leaf spring plane to the next leaf spring plane via the fixation zones 9.

However, the load paths are inherently abrupt with highly localized transfer of load through the sparsely placed fixation zones 9. There are even instances where the entire load is carried through a single fixation zone 9 in the center of the disc. The abrupt load paths can lead to high stress regions, which can detract from the appropriate biomechanical performance, i.e., strength, flexibility, and range-of-motion, of the prosthesis.

The need exists for a prosthetic disk which is easy to manufacture and provides the proper balance of flexibility and stability through improved load distribution.

SUMMARY

Accordingly, the present disclosure relates to an intervertebral prothesis dimensioned for insertion within an intervertebral space between adjacent vertebrae to replace at least a portion of an intervertebral disc removed therefrom. The prosthesis includes a disc member defining a longitudinal axis and a lateral axis transverse to the longitudinal axis. The disc member includes an exterior wall portion having a first slit with a longitudinal component of direction and a second slit with a lateral component of direction. The first and second slits are dimensioned to extend sufficiently within the exterior wall portion and are arranged whereby upon insertion of the disc member within the intervertebral space forces exerted on the disc member are transferred through the slit arrangement along the exterior wall portion. Preferably, the first slit extends in a general longitudinal direction and the second slit extends in a general lateral direction. The disc member may include an interior cavity disposed within the exterior wall portion with the first and second slits extending through the exterior wall portion in communication with the interior cavity.

The disc member may include first and second support surfaces disposed at respective longitudinal ends of the disc member and being dimensioned to supportingly engage vertebral portions of respective vertebrae. At least one of the first and second support surfaces has an opening extending therethrough in communication with the interior cavity. A pair of end caps can also be provided.

In a preferred embodiment, the disc member includes a plurality of lateral slits extending in a general lateral direction and a plurality of longitudinal slits extending in a general longitudinal direction whereby at least two of the lateral slits are longitudinally displaced relative to the longitudinal axis and disposed in at least partial overlapping relation. At least one of the longitudinal slits preferably extends between and interconnects the two lateral slits. Preferably, at least three lateral slits are longitudinally displaced relative to the longitudinal axis and arranged to define overlapping portions.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiment(s) of the present disclosure are described herein with reference to the drawings wherein:

FIGS. 1-3 illustrate a prior art intervertebral disc prosthesis;

FIG. 4 is a perspective view of the intervertebral disc prosthesis in accordance with the principles of the present disclosure;

FIG. 5 is a view illustrating a portion of the vertebral column;

FIG. 6 is a view take along the lines 6--6 of FIG. 5 illustrating the intervertebral prosthesis of FIG. 4 positioned within the intervertebral space defined between adjacent vertebrae;

FIG. 7 is a perspective view of an alternate embodiment of the artificial intervertebral disc prosthesis;

FIG. 8 is a perspective view of another alternate embodiment of the intervertebral disc prosthesis;

FIG. 9 is a perspective view of an alternate embodiment of the invertebral disc prosthesis having a pair of end caps; and

FIG. 10A is a cross-sectional view taken through the vertebral body to illustrate a top view of the fusion cage of the present disclosure; and

FIG. 10B is a perspective view of the fusion cage of FIG. 10A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, in which like reference numerals identify similar or identical elements throughout the several views, and referring in particular to FIG. 4, the artificial intervertebral prosthesis of the present disclosure is illustrated. Intervertebral proses 100 is intended to replace part or al of the supporting function of a diseased intervertebral disc which had been previously removed through a discectomy procedure or the like. Intervertebral prosthesis 100 is advantageously dimensioned to be positioned between adjacent vertebrae in supporting contacting relation with the vertebral end plates thereof to maintain the adjacent vertebrae in appropriate spaced relation while resting the natal biomechanics (e.g., including stiffness, mange of motion, and load carrying capacity) of the spinal or vertebral segment.

Intervertebral prosthesis 100 includes a single component, namely, disc or body member 102. Body member 102 is in the general shape of an intervertebral disc (e.g., kidney-shaped) as shown and defines longitudinal axis "a" extending the height of the member 102 and radial oral) axis Abe generally transverse to the longitudinal axis. An angular reference is defined by "c" as shown in FIG. 4. Body member 102 includes an exterior wall 104 having cannulated bore (interior cavity) 106 defined therein which extends the height of body member 102 in general concentric relation with the longitudinal axis "a". Body member further includes upper and lower longitudinally opposed support surfaces 108, 110 which supportingly engage the respective end faces of the adjacent vertebrae upon insertion of the prosthesis. Support surfaces 108, 110 are each arcuate in configuration defining a slight outer curvature which preferably cords to the slight inward curvature of the vertebral end plates so as to facilitate positioning and retention of the prosthesis within the intervertebral space.

With continued reference to FIG. 4, exterior wall 104 includes a plurality of slits 112 defined therein consisting of lateral (radial) slits 112a and longitudinal slits 112b connecting later